The term “in-line” refers to continuous NDT/NDI which occurs within the production line in which an alloy is produced or finished. Testing occurs continuously as the final product emerges from the production line, and its purpose is to ensure that the surface of the high value alloy is smooth, with no flaws, cracks or scratches. The bars to be inspected are generally cylindrical, the length of the axis of the cylinder usually being much larger than its diameter. Hereafter, a direction parallel to the axis of the bar will be referred to as an “axial” direction, and a direction perpendicular to the axis of the bar will be referred to as a “radial” direction. The bar cylinders may be comprised of solid material throughout, or they may be in the form of a tube or pipe with an outer diameter and an inner diameter. Testing is usually carried out by placing one or more probes close to the surface of the bar to be inspected, and then rapidly rotating the bar about its axis while translating either the bar or the probes in an axial direction. The purpose of the rotation and translation is to ensure that all parts of the surface to be tested pass sufficiently close to one or more probes so that flaws may be detected with sufficient sensitivity.
Use of eddy current probes for near-surface inspection is well known in the art. The probe may be in the form of a single coil, or there may be an array of coils placed at different orientations to enhance the sensitivity and efficiency of defect detection. The amplitude of the signal detected by a probe due to a defect is a sensitive function of the distance between the probe and the inspected surface. This distance is commonly referred to as the “lift-off” distance. In order to ensure that defects of the same type and size result in the same detected signal amplitude, it is essential to maintain constant lift-off at all times.
A significant difficulty in the prior art is maintaining constant lift-off under conditions of in-line testing, in which there is rapid and continuous relative movement between the probe and the inspected surface. The problem is exacerbated by the fact that the bar being tested is often not perfectly round and/or the rotation of the bar is eccentric. In either case, maintaining constant lift-off requires that the probe position needs to move rapidly back and forth in the radial direction as the bar rotates.
One prior art solution has been to employ a spacer between the probe and the surface, and to apply pressure on the spacer, often with the entire weight of the probe and mechanism assembly, or possibly by means of some other external force. The pressure ensures that the spacer always remains in intimate contact with the rotating surface, but the probe must be allowed to move in order to take account of the bar being out-of-round or the rotation being eccentric.
There are multiple problems with this prior art approach. If the spacer is made of hard material, then the pressure is liable to damage the inspected surface by causing scratches. On the other hand, if the spacer is made of soft material, then the high speed relative motion between the soft material and the alloy surface will result in rapid wear of the soft spacer. As the spacer wears down, the lift-off will change, resulting in detection errors until the spacer can be replaced during a maintenance procedure which will disrupt production.
Another problem with the prior art solutions is that the motion of the probe as the bar rotates needs to be accurately aligned with the radial direction of the bar. Any rotation of the probe, or translation parallel to the surface of the bar, will lead to defect detection errors.
Yet another problem with prior art solutions is that the probe holder is customized for a particular bar diameter. This means that the probe holder must be replaced and/or re-adjusted every time the bar size is changed, which is often a labor intensive operation resulting in costly disruption to production.
Yet another problem with prior art solutions is that the probe holder often does not allow the probe to be used in good alignment to the very end of a bar. This means that each bar will have an area at its ends which was not inspected for surface quality.